1. Field of the Invention
The present invention relates to a magnetic disk drive. The invention in particular relates to an automatic head retract mechanism of immediately retracting a magnetic head, when the magnetic disk drive becomes unable to receive power from a power supply due to a power failure or the like during operation. 2. Related Art
A magnetic disk drive can become unable to receive power from a power supply unexpectedly when, for example, a malfunction is caused by a user or a power failure occurs during operation. When this happens, a magnetic head should be retracted away from a magnetic disk immediately, to prevent the magnetic head from touching the magnetic disk. To retract the magnetic head, power needs to be supplied to an actuator for driving the magnetic head. However, such power cannot be drawn from the power supply.
Unexamined Japanese Patent Application Publication No. 2001-307408 discloses a technique of supplying induced power generated in a motor which rotates a magnetic disk, to an actuator to retract a magnetic head. The magnetic disk keeps rotating by an inertial force for awhile after power from a power supply stops. This makes the motor function as a power generator. The above technique exploits this property.
FIG. 1 shows a construction of a magnetic disk drive disclosed by Unexamined Japanese Patent Application Publication No. 2001-307408.
A spindle motor 51 is a three-phase AC (alternating current) motor for rotating a magnetic disk. A SPM drive circuit 52 supplies driving power to the spindle motor 51.
A voice coil motor 76 is an actuator for driving a magnetic head. The voice coil motor 76 is supplied with power from a VCM drive circuit 71 during a normal operation period (i.e. when the magnetic disk drive receives power from a power supply normally), and from the spindle motor 51 during a retraction period (i.e. when the magnetic disk drive cannot receive power from the power supply and the magnetic head needs to be retracted). A switch circuit 53 controls switches 78 and 79, to switch between the VCM drive circuit 71 and the spindle motor 51.
A rectification circuit 75 rectifies three-phase AC induced power generated in the spindle motor 51. A control circuit 77 controls switches included in the rectification circuit 75.
A voltage in each phase of the induced power has a near-sinusoidal waveform. These three phases differ by an electrical angle of 120°. In other words, a phase having a highest voltage out of the three phases differs with time. For efficient rectification, a maximum detection circuit 72 controls switches 65, 66, and 67 so that only a switch corresponding to a highest-voltage phase of the induced power is closed. Meanwhile, a minimum detection circuit 73 controls switches 68, 69, and 70 so that only a switch corresponding to a lowest-voltage phase of the induced power is closed.
In this way, the magnetic disk drive rectifies the induced power generated in the spindle motor 51 and supplies the rectified power to the voice coil motor 76.
In recent years, magnetic disk drives are becoming increasingly smaller. In keeping with this trend, semiconductor chips on which the above circuits are integrated need to be downsized too. In the case of FIG. 1, integrating the switches 78 and 79 in a semiconductor chip requires two pairs of power transistors, i.e., a total of four power transistors. A power transistor handles a large amount of current and therefore has a large volume. Hence it is desirable to use fewer power transistors to realize a smaller semiconductor chip. In view of this, the inventors of the present application considered the following construction in the process of development.
FIG. 2 shows a construction of a magnetic disk drive considered by the inventors of the present application.
In the drawing, an output terminal 58 of the rectification circuit 75 is connected to a power input terminal of the VCM drive circuit 71, and a switch 57 is provided between the terminal 58 and a power supply.
The switch 57 is closed during the normal operation period, and opened during the retraction period. In so doing, power is supplied to the VCM drive circuit 71 both during the normal operation period and the retraction period. This construction enables the number of power transistors used for power source switching to be reduced to one (i.e. the switch 57).
This magnetic disk drive serves the need for a smaller semiconductor chip. When the magnetic disk drive is put to use, however, the following new problem arises.
The maximum detection circuit 72 operates to control the switches 65 to 67 only in the retraction period, and does not operate in the normal operation period. This is because if the maximum detection circuit 72 closes any of the switches 65 to 67 during the normal operation period, a DC voltage of the power supply reaches the spindle motor 51, thereby interfering with the operation of the spindle motor 51.
In the retraction period, the maximum detection circuit 72 uses the induced power generated in the spindle motor 51, to control the switches 65 to 67. In general, a voltage of induced power is relatively low. Therefore, the maximum detection circuit 72 steps-up the voltage of the induced power and uses the stepped-up voltage to control the switches 65 to 67, to bring one of the switches 65 to 67 into full conduction.
To step-up the induced power, however, it is first necessary to charge a capacitor in the maximum detection circuit 72. This means the maximum detection circuit 72 cannot immediately obtain a sufficiently stepped-up voltage to control the switches 65 to 67 when the retraction period begins. Due to the recent trend toward smaller magnetic disk drives, induced power tends to be smaller. Under such circumstances, it is difficult to design magnetic disk drives that can reliably retract a magnetic head upon a power failure or the like.